Origin of open clusters revealed by the evolution of the m_max$-$M_ecl relation

Abstract

Using the Gaia DR3 open cluster catalog, we identified the most massive star in each observed cluster. Examining the m_max$-$M_cluster relations across different age ranges, we find that as clusters age, the relation gradually deviates from the initial m_max$-$M_ecl relation and eventually exhibits clear age stratification. We conducted N$-$body simulations for both individual cluster evolution and subcluster coalescence. Four gas expulsion modes were tested for individual clusters, and two scenarios were modeled for cluster coalescence. Under all four gas expulsion modes, the evolution of the m_max$-$M_cluster relation follows a similar trajectory, differing mainly in evolutionary speed. The coalescence simulations show comparable behavior but align better with the observations, as both exhibit systematically lower m_max$-$M_cluster relations than individual cluster simulations. This systematically lower observed m_max$-$M_cluster relation suggests slower cluster mass loss and smaller masses for the most massive stars$-$both conditions reproduced in the coalescence simulations. Observations also show that clusters older than 5 Myr have most massive stars significantly deviating from the initial m_max$-$M_ecl relation. From this perspective, the coalescence simulations also provide a better match to the observations. In conclusion, the evolution of the m_max$-$M_ecl relation supports subcluster coalescence as a dominant pathway for open cluster formation, consistent with our previous work.

Figures (9)

• Figure 1: Comparison of the $m_{\rm max} -M_{\rm cluster}$ relations in the cluster coalescence and individual cluster simulations shown in Table.\ref{['tab1']}. The timestep is linearly spaced at 0.25 Myr.
• Figure 2: The $m_{\rm max} -M_{\rm cluster}$ relation of the observed open clusters evolves over time. The observational and theoretical $m_{\rm max} -M_{\rm ecl}$ relations in equation.\ref{['eq:mmaxmecl2']} and Zhou2024PASP-2 correspond to the cyan and black dashed curves, respectively. "o" and "m" are the bound open clusters and unbound moving groups in the catalog of Hunt2024-686.
• Figure 3: Comparison of the $m_{\rm max} -M_{\rm cluster}$ relations in the cluster coalescence and individual cluster simulations within 5-100 Myr. Panels (a)-(d) show the $m_{\rm max} -M_{\rm cluster}$ relations for four different gas expulsion modes in the individual cluster simulations, which are combined in panel (g). Panels (e) and (f) show the $m_{\rm max} -M_{\rm cluster}$ for two scenarios in cluster coalescence simulations, which are merged in panel (h). Panels (g) and (h) are combined in panel (i). The timestep is linearly spaced at 0.25 Myr.
• Figure 4: Comparison between the observed tidal radius ($r_{\rm t,obs}$) and $r_{\rm 50,obs}$ (the radius containing 50% of cluster members) from the catalog of Hunt2024-686, and the theoretical tidal radius calculated using equation.\ref{['tide']} with the cluster masses from the same catalog.
• Figure 5: The $m_{\rm max} -M_{\rm cluster}$ relation of the observed open clusters within 5-100 Myr. (a) Same as Fig.\ref{['mMo']}; (b) Restricting the members of the observed cluster to those within the theoretical tidal radius $r_{\rm t}$, and then selecting the most massive star; (c) Same as panel (b), but further requiring $r_{\rm t}/r_{\rm 50,obs} > 1.69$.